Direct exfoliation of graphene in ionic liquids with aromatic groups

Colloids and Surfaces A: Physicochem. Eng. Aspects 463 (2014) 63–69

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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa

Direct exfoliation of graphene in ionic liquids with aromatic groups Rozana Bari a , George Tamas b , Fahmida Irin a , Adelia J.A. Aquino b , Micah J. Green a,∗ , Edward L. Quitevis b,∗∗ a b

Department of Chemical Engineering, Texas Tech University, Lubbock, TX 79409, United States Department of Chemistry & Biochemistry, Texas Tech University, Lubbock, TX 79409, United States

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Imidazolium

ionic liquids with phenyl groups disperse graphene noncovalently without stabilizers. • The graphene concentration is higher in diphenyl than in monophenyl substituted ionic liquids. • DFT calculations show phenylgraphene ␲–␲ interactions help stabilize graphene–IL dispersions.

a r t i c l e

i n f o

Article history: Received 3 July 2014 Received in revised form 8 September 2014 Accepted 12 September 2014 Available online 20 September 2014 Keywords: Graphene Ionic liquid ␲–␲ stacking Conductivity

a b s t r a c t Novel ionic liquids (ILs) were designed and synthesized to contain aromatic groups on the imidazolium cation that non-covalently interact with graphene surfaces. This route enables the dispersion of pristine graphene without covalent functionalization or an additive stabilizer; such dispersions are stable against aggregation and display high concentration values. We find that ILs without these aromatic groups are less effective in graphene dispersion, and the dispersed graphene concentration increases with increasing interaction between the cation and graphene surface. Density functional theory (DFT-D3) calculations support the experimental observations and provide a foundation for predictive modeling of IL design for optimal graphene dispersions. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Graphene (i.e. single-layer graphite sheets) is a form of twodimensional sp2 hybridized carbon [1,2]. The unique properties of graphene include mechanical strength, electrical conductivity, and thermal conductivity [1–3], which collectively hold promise for use in a wide range of applications, including composites [4], thin

∗ Corresponding author. Current address: Department of Chemical Engineering, Texas A&M University, College Station, TX 77845, United States. ∗∗ Corresponding author. Tel.: +1 806 834 3066; fax: +1 806 742 1289. E-mail addresses: [email protected] (M.J. Green), [email protected] (E.L. Quitevis). http://dx.doi.org/10.1016/j.colsurfa.2014.09.024 0927-7757/© 2014 Elsevier B.V. All rights reserved.

conductive films [5,6], and supercapacitors [7,8]. Graphene was first identified by micromechanical cleavage of graphite in 2004 [2], but other techniques have been developed to increase graphene production scalability. In the current paper, we focus on the major problem of producing graphene from graphite using liquid phase exfoliation without dispersants or oxidation [9,10]. Graphene layers are held together by van der Waals forces in graphite; this creates problems both in exfoliating graphene from graphite as well as keeping graphene dispersed in a liquid medium rather than aggregating. The most common approach to solving this problem of producing graphene from graphite involves oxidizing the graphite to produce graphite oxide, which may then be exfoliated to yield dispersions of graphene oxide. Graphene oxide (GO) may subsequently be chemically or thermally treated to remove the

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Scheme 1. Generic synthesis of the asymmetric ILs.

covalent functional groups and produce reduced graphene oxide (rGO) [6,11–15]. Alternatively, graphene may be produced through exfoliation (through sonication or high shear) of graphite powder in a solution of stabilizer molecules such as micelle-forming surfactants, polymers, and aromatic hydrocarbons [16–18]. (A summary of these methods is given in Table S1 in the Supplementary Materials.) However, it is highly desirable to directly disperse graphene in liquids with no stabilizer or additive of any kind, because such additives may have adverse effects on the mechanical or electrical properties of graphene-based films, sensors, or composites. Only a few candidate solvents have been identified for such direct exfoliation and dispersion [19], and one of the most promising are ionic liquids [20]. Ionic liquids (ILs) are organic molten salts with melting point temperatures below 100 ◦ C, which have been used extensively as solvents. ILs are non-volatile [21], thermally stable [22], nonflammable, recyclable [23], and capable of dissolving a range of solutes. Furthermore, their properties (such as miscibility and viscosity) can be tuned via chemical changes to the cation or anion [24]. Additionally, ILs are attractive because of their status as green solvents due to their low vapor pressures and ease of recycling, in contrast to common organic solvents [9]. Prior reports have indicated that ILs can disperse graphene directly [25,26]. The mechanisms behind IL–graphene interactions are just beginning to be understood [27]. However, the existing literature gives little insight into effective IL design for graphene dispersion. In the present investigation, we produce novel ILs with controlled differences in chemical functionality and demonstrate that these ILs can be used to directly disperse graphene at high concentrations. 2. Materials and methods 2.1. Materials Expanded graphite (EG) was graciously provided by Asbury Carbons (CAS# 7782-42-5, Grade 3772). All the other chemical reagents and solvents were purchased from commercial sources (Sigma–Aldrich, Acros Organics, 3M) and were used as received. All reactions were run using oven-dried glassware under nitrogen atmosphere. 1 H, 13 C and 19 F NMR spectra were recorded on JEOL 400 spectrometer and collected as solutions of deuterochloroform (bromide ionic liquids) or deuteroacetone (bistriflate ionic liquids). Four different types of ILs were synthesized: 1-benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([BnzC1 im] [NTf2 ], IL-1), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([bmim][NTf2 ], IL-2), 1-benzyl-3-methylimidazolium bromide ([BnzC1 im][Br], IL-3), and 1,3-bis(phenylmethyl) imidazolium bis(trifluoromethylsulfonyl)amide ([(Bnz)2 im][NTf2 ], IL-4). Their chemical structures are presented in Table 1. The synthetic procedures of the ILs have been published elsewhere [28–30]. They follow Scheme 1 for the asymmetric compounds and Scheme 2 for the symmetric compound. 2.2. Preparation of graphene dispersions Expanded graphite (10 mg mL−1 ) was added to the IL and then tip-sonicated at an output wattage of 10 W for 1 h in an icewater bath. The resulting dispersion was then centrifuged (Centrific

Centrifuge 225, Fischer Scientific) at ∼5000 rpm for 6 h to remove large aggregates. The supernatant was collected and used for further characterization. Four different ILs varying in chemical composition (anion and cation) were investigated with the final graphene yields summarized in Table 1. The supernatant containing graphene in IL-4 was collected as shown in the inset of Fig. 1a. The concentration of graphene was measured by vacuum filtration. Dichloromethane was used to wash the excess IL from the filter paper, and the weight difference was used to calculate a graphene concentration. The absorbance was measured by a Shimadzu UV–vis spectrophotometer 2550 in the 200–800 nm wavelength range. The Lambert-Beer law, A = ˛LC, where the absorbance A is proportional to the product of concentration C and path length L, was utilized to measure the extinction coefficient ˛ of the IL stabilized graphene dispersion. The pure IL was used as a blank to eliminate the background effect. The absorbance spectrum of graphene dispersion is shown in Fig. S2 in the Supplementary Materials, and the extinction coefficient was computed to be 3181 mL mg−1 m−1 for IL-4. The value of the extinction coefficient is comparable to that of another system reported in literature (˛ = 1172 mL mg−1 m−1 for graphene in l-hexyl-3methylimidazolium hexafluorophosphate) [25].

2.3. Characterization of graphene/ionic liquid dispersions To determine the surface area of EG, physisorption analysis was performed. An Autosorb iQ machine (Quantachrome Instruments) was utilized to conduct the experiment. Initially EG was degassed for 15 h at 90–100 ◦ C. After degassing, the nitrogen adsorption–desorption isotherm was acquired. The surface area of the sample was determined by applying the multipoint Brunauer–Emmett–Teller (BET) method to the isotherm. In order to measure the degree of exfoliation, Raman spectroscopy was used. A Thermo ScientificTM DXR Raman microscope was utilized. The filtered sample was used to measure the Raman spectrum using a 532 nm laser. To measure the number of layers of graphene in a typical dispersed flake, transmission electron microscopy (TEM) was performed. The TEM sample was prepared by drop-casting the dispersion on a lacey carbon coated 200-mesh copper grid and then washing with ethanol to remove the IL. A Hitachi H8100 TEM was utilized to image the sample with the accelerating voltage set as 75 kV. To confirm that the prepared sample is graphene, X-diffraction (XRD) was performed on the vacuum filtered film. The instrument used for the analysis of the graphene sample was a Rigaku MiniFlex II powder diffractometer with a Cu source and a scintillation detector. The X-ray source was operated at a power setting of 30 kV and 15 mA. The data was collected with a step size of 0.005◦ and a collection time of 4 s per step. In order to quantify the atomic percentage of the elements in the vacuum filtered film of graphene and IL-4, X-ray photoelectron spectroscopy (XPS) was performed with a 5000 VersaProbe Electron Spectroscopy for Chemical Analysis (ESCA) spectrometer. A focused monochromatic X-ray beam of 1486.6 eV was utilized to do this characterization. Four different runs were performed on the film at different spots. The thickness of the graphene film on filter paper was measured by scanning electron microscope (SEM) (Hitachi S-4300). The filtered sample was mounted on double sided carbon tape and the accelerating voltage set at 2 kV. The electrical resistivity was measured by the standard twopoint probe method. The vacuum filtered sample was used to measure the resistivity by a high resistance meter (Model-HR2, AlphaLab, Inc.). A number of measurements were performed on

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Table 1 List of ionic liquids tested for stable dispersion of graphene. Structure

Stability of the dispersion after centrifugation

Concentration (mg mL−1 )

Ionic liquid

Chemical name

IL-1 [BnzC1 im][NTf2 ]

1-Benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide

Stable dispersion

0.081

IL-2 [bmim][NTf2 ]

1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide

Unstable dispersion

–

IL-3 [BnzC1 im][Br]

1-Benzyl-3-metylimidazolium bromide

Unstable dispersion

–

IL-4 [(Bnz)2 im][NTf2 ]

1,3-Bis(phenylmethyl)imidazolium bis(trifluoromethylsulfonyl)amide

Stable dispersion

5.8

Scheme 2. Synthesis of the symmetric IL.

different positions of the film and the statistical average of the conductivity (as calculated from resistance) was reported. 2.4. Density functional theory (DFT) calculations In view of the large size of the molecular systems to be studied, density functional theory was used with the Perdew, Burke and Ernzerhof (PBE) functional [31]. For computational efficiency, the

resolution-of-the-identity (RI) approach was employed to speed up the computation through effective calculation of the two-electron integrals [32]. The dispersion interaction was accounted for by means of the D3 dispersion correction of Grimme et al. [33]. The computations were performed using the triple-zeta valence plus polarization (TZVPP) basis set [34]. All calculations were carried out by means of the Turbomole program suite (version 6.5) [35].

Fig. 1. TEM images of (a) graphene flakes stabilized by IL-4, and (b and c) the magnified view of the flake edges, indicating 2–5 layers of sheets. The inset of (a) shows a graphene/ionic liquid dispersion which is stable after centrifugation.

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3. Results and discussion 3.1. Effect of ionic liquid on dispersion We compared the ability of the various ILs to disperse graphene as a function of their chemical composition. Our results are summarized in Table 1. IL-2 and IL-3 did not disperse graphene at measurable levels. However, both IL-1 and IL-4 did successfully disperse graphene, and notably, IL-4 yielded an unusually high graphene concentration of 5.8 mg mL−1 . These results indicate a much stronger interaction of graphene with IL-4 than with IL-1. This is correlated with the presence of two phenyl groups in IL-4 and one in IL-1. Our prior work indicates that aromatic hydrocarbons undergo non-covalent ␲–␲ stacking interactions with graphene [18]. In this case, the two phenyl groups in IL-4 increases the interaction between graphene and IL-4. Graphene was not dispersed when IL-2 was used. This may be associated with the lack of aromatic groups in this IL. The cation in IL-2 contains a butyl group in contrast with the phenyl group in IL-1. Wang et al. [26] reported graphene dispersion up to 0.95 mg mL−1 using IL-2 [26]. The difference between their results and ours may be attributed to several factors. First, the high sonication power (750 W) used by Wang et al. may have caused some reductions in the average sheet lateral size, which is correlated with higher concentrations [36]. A more likely cause is that insufficient centrifugation (only 20 min in their case) and this experimental error in measuring the sediment rather than the supernatant introduced error into their estimates. It should be noted that a 47.5% yield as they report is unprecedented and quite unlikely for sonication-based exfoliation. Another paper reported high concentrations (5.33 mg mL−1 ) for 1-hexyl-3-methylimidazolium hexafluorophosphate similar to ours (5.8 mg mL−1 in IL-4), but for extremely high (24 h) sonication times [25]. Again, high sonication times are known to decrease graphene sheet lateral size, resulting in higher yields at the cost of smaller sheets [37]. The difference between IL-1 and IL-3 lies in the anion. The bromide anion in IL-3 causes a sharp increase in viscosity such that exfoliation via sonication becomes ineffective; this is a practical concern that prevents us from examining the actual cation–graphene interactions in the case of IL-3. 3.2. Characterization of graphene dispersion TEM was performed on the graphene dispersion in IL-4 to access the number of layers in the graphene sheets. The TEM images are depicted in Fig. 1 and the inset of Fig. 1a shows the IL-4 stabilized graphene dispersion. The magnified view of the flake (Fig. 1b and c) indicates that there are 2–5 layers of graphene sheets present. Folded graphene flakes are observed in Fig. 1a. The TEM shows that the lateral sheet dimension of graphene varies from ∼300 nm to 1200 nm. Additional TEM images are available in the Supplementary Materials (Fig. S3). To further characterize the graphene dispersion in IL-4, Raman spectroscopy on a vacuum-filtered film was utilized. In the spectrum plotted in Fig. 2, we observe a G peak (associated with sp2 hybridization, common to graphene) at ∼1600 cm−1 and a 2D peak at ∼2700 cm−1 . The shape of the 2D peak is characteristic of graphene rather than graphite, confirming the presence of stable, exfoliated graphene in the dispersion [16,38]. The number of layers of graphene can be determined by comparing the 2D peak from the Raman spectrum with the 2D peak of the Raman spectra presented by Ferrari et al. and comparison of the 2D peaks indicates that 1–5 layers of graphene present [38]. This result supports the observation from the TEM images as the TEM images indicate that 2–5 layers of graphene sheets present. Near 1330 cm−1 an additional peak (D peak) is observed that is associated with

Fig. 2. Raman spectra of graphene in IL-4. The G-peak and 2D peak (without graphite-like shoulder) confirm graphene exfoliation in IL-4.

Fig. 3. XRD data on graphene in IL-4 vacuum filtered film shows graphene’s characteristic peak at 26.48◦ .

graphene edges in sonicated pristine graphene dispersions [17]. XRD on graphene in IL-4 in Fig. 3 shows the characteristic peak of graphene at 26.48◦ which is associated with the (0 0 2) plane [39]. Moreover, the absence of graphitic peaks at 45◦ and 55◦ confirms that the sample is graphene rather than graphite [40]. Peaks at 17.95◦ (as shown in the inset of Fig. 3) and 31.49◦ are characteristic of the PTFE filter paper [41,42]. Also, XPS was carried out to determine the presence of residual ionic liquid associated with the graphene surface (Fig. 4). The atomic percentages measured from several locations on the film from XPS are 79.0%, 13.1%, 6.6%, 1.0%, and 0.3% for carbon, fluorine, oxygen, nitrogen, and sulfur, respectively. These values cannot be taken as quantitative ratios because

Fig. 4. XPS spectra of a vacuum-filtered graphene film cast from a graphene/IL-4 dispersion. The major peaks for C, F, O, N, and S are shown.

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Fig. 5. SEM images of (a) graphene film in IL-4 on filter paper (inset) prepared by vacuum filtration, with a (b) magnified view of the film showing graphene flakes. The thickness of the film is ∼6 ␮m and the electrical conductivity of the film is measured to be 1.07 × 10−2 S/m.

of the variable graphene and ionic liquid content on the PTFE filter itself. However, the presence of sulfur, oxygen and nitrogen does indicate that the [NTf2 ]− anion and imidazolium cation are present in the sample. This confirms the strong non-covalent association of graphene with the IL even after vacuum filtration. The electrical conductivity of this same vacuum-filtered graphene film was measured to be 1.07 × 10−2 S/m. The SEM images of the top surface of the film are depicted in Fig. 5 and the inset of Fig. 5a shows the vacuum filtered film on a filter paper. Additional images are available in Supplementary Materials (Fig. S4). The thickness of the film was measured from the SEM (Fig. 5a) as ∼6 ␮m. Individual graphene flakes are visible in the magnified view of the film (Fig. 5b).

The binding energies of the graphene–cation and graphene–anion complexes were calculated as the difference between the energies for the complex and the individual structures (i.e. E = E(complex) − [E(ion) + E(graphene)]) and are presented

3.3. DFT model calculations The structure of the polyaromatic hydrocarbon (PAH) C57 H19 , which was used as the graphene model in the calculations, is shown in Fig. 6. The cations and the anion of three ionic liquids were computationally investigated and their binding energies to graphene were calculated: [C1 C1 im]+ , [BnzC1 im]+ (IL-1), [(Bnz)2 im]+ (IL-4), and [NTf2 ]− (Fig. 6). Besides the cations of IL-1 and IL-4, which were experimentally studied here, calculations on the [C1 C1 im]+ cation give information about the contribution of the imidazolium ring to the overall binding of the ILs on the surface of the graphene. The ␲–␲ stacking interaction between the aromatic systems and the PAH, is characterized by intermolecular bond distances and interaction energies. Functionalizing ionic liquids with phenyl rings, contributes to a better adsorption (i.e. higher binding energy) of the salts on the surface of the two-dimensional array of carbon atoms which, it can be argued, will have a direct effect on the exfoliation and stability of graphene under sonication conditions.

Fig. 6. Top view of the C57 H19 polyaromatic hydrocarbon system used as a graphene model.

Fig. 7. Graphene and IL interaction computed at the PBE-RI/TZVPP level: (a) [C1 C1 im]+ cation and graphene, (b) IL-1 [BnzC1 im]+ cation and graphene, (c) IL-4 [(Bnz)2 im]+ and graphene, and (d) [NTf2 ]− anion and graphene. The two additional aromatic rings on IL-4 provide an increased interaction with graphene relative to structure (a). Shorter distances between the cation and graphene can be observed and also a slight curvature of the carbon sheet as an effect of stronger ␲–␲ interactions.

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in Fig. 7 (see Tables S2 and S3 in Supplementary Materials for total energies). Selected intermolecular distances of the optimized structures are also presented in Fig. 7. In the absence of aromatic substituents, the dimethylimidazolium ring (Fig. 7a) adopts a parallel-displaced ␲–␲ stacking type of structure with a calculated binding energy of −27.4 kcal/mol. Insertion of the first benzyl group increases the cation’s degree of adsorption on the graphene by −1.3 kcal/mol, due to an additional favorable interaction of the aromatic ring twisted into a T-shaped configuration (Fig. 7b). The strongest interaction was computed for the di-benzyl substituted cation. The binding energy determined was the highest (i.e. −30.6 kcal/mol), and correlates well with the two phenyl groups aligned approximately parallel to the plane of the graphene flake thereby enhancing ␲ stacking interactions, with a shorter distance between the cation and graphene than in the case of the mono-benzyl substituted cation and also with a slight curvature observed on the carbon sheet (Fig. 7c). The anion–graphene interaction (Fig. 7d) was −11.7 kcal/mol, roughly a third of the strongest graphene–[(Bnz)2 im]+ interaction, rendering the graphene–cation complex as the main contributor to the overall IL–graphene binding energy and pointing to the importance of the aromatic functionalization of toe ILs for dispersing graphene. 4. Conclusions In conclusion, we used various ILs to exfoliate and stabilize high concentrations of pristine graphene sheets. Among the four different ILs, IL-4 with two phenyl groups was the most effective one in stabilizing the graphene dispersion with a high yield of 5.8 mg mL−1 . The most remarkable achievement was the dispersion of graphene in ILs at high concentrations without covalent functionalization or additive stabilizer. These preliminary computational calculations suggest that the addition of ␲–␲ stacking capable substituents will increase the adsorption of the ionic liquids on the graphene surface with a possible favorable effect on the exfoliation and stabilization processes. More extensive computational studies involving different levels of theory are needed to obtain more accurate values of the binding energies of the various cation–graphene complexes. Nevertheless, the theoretical results reported herein follow the experimental trend, confirming that computational chemistry is a useful tool in developing these ‘designer solvents’ for applications in graphene dispersion. Acknowledgements For the Raman measurements, we thank Nick Wietfeldt from Xolve, Inc, as well as Colin Young and Professor Matteo Pasquali of Rice University. TEM and SEM were performed at the TTU Imaging Center with help from Dr. Mark J. Grimson, Dr. Callum Hetherington, Mary Catherine Hastert, and Dr. Lauren Gollahon. For the XRD experiment we thank Dr. Daniel Unruh, the crystallographer in the Department of Chemistry and Biochemistry, TTU. For the surface area measurements and the XPS characterization we thank Dr. Juliusz Warzywoda and Dr. Al Sacco of TTU. We also wish to thank Dr. Sriya Das and Dorsa Parviz for their advice. We thank the Department of Chemistry and Biochemistry at TTU for use of the Robinson cluster whose purchase was funded by the National Science Foundation under CRIF-MU instrumentation grant CHE-0840493. Funding was provided to M. J. Green by the National Science Foundation under CAREER award CMMI-1253085 and by the Air Force Office of Scientific Research Young Investigator Program (AFOSR FA9550-11-1-0027) and to E. L. Quitevis by the National Science Foundation under Grant No. CHE-1153077. A. J. A. Aquino was supported by the Robert A. Welch Foundation under grant No. D-0005. We thank the National Science Foundation for

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[26]

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